It is well known in the art of structural composite materials to employ a sheet of expanded cellular foam as a core of a sandwich panel comprising outer layers of a fibre reinforced matrix resin composite material. The sandwich panel is typically manufactured by disposing respective fibre layers on opposite surfaces of the foam sheet and then infusing a curable resin into the fibre layers and against the opposite surfaces during a vacuum assisted resin transfer moulding step. The resin is then cured to form the sandwich panel.
There is a need to provide a strong adhesive bond between the cured resin layers and the core, so that there is a high peel strength between the cured resin and the core.
There is also a need to minimise the resin take-up of the foam core. This adds undesired weight to the sandwich panel. The opposite surfaces of the cellular foam core tend to have a propensity to take-up the curable resin by absorption of the resin into the opposite surfaces, when the resin is infused against the surfaces during a vacuum assisted resin transfer moulding step.
There is also a need for the foam core of the sandwich panel to exhibit high mechanical properties such as high compressive strength and high shear strength, with uniform mechanical properties over its surface area.
In combination, there is a need for the sandwich panel to exhibit a combination of mechanical properties and low resin uptake.
It is known to treat the surfaces of a foam core by a thermal sealing treatment. For example, US-A-2005/0182239 discloses a process for producing moulded poly(meth)acrylamide foams in which heat and pressure are applied to the foam surface in order to compact the surface. The surface-compacted foam is thereby sealed and can be used as a removable core in fibre-composite components. It is stated that the surface-compacted foam exhibits reduced resin absorption for the same adhesion when used as a core. During the pressing operation, a press is heated to a temperature close to the foaming temperature, the cold foam is inserted into the heated press, the press is closed to apply a contact pressure, stated to be ideally about 30% of the compressive strength of the foam, the heated surfaces of the foam are compressed whereas the cold inner regions of the foam are not compressed and after the desired final thickness is achieved the press and the foam within the press are allowed to cool while the press is closed, the foam only being removed from the press after becoming sufficiently cold to be dimensionally stable after removal. This specification discloses providing a removable core by avoiding adhesion to the core surface, whereas for structural sandwich panels it is generally desired to have a strong adhesive bond, exhibiting high peel strength, between the core and the composite material laminated thereto.
High performance composite sandwich panels have traditionally been constructed from honeycomb materials and structural cellular polymer foams made from polyvinylchloride (PVC) and styrene acrylonitrile (SAN) polymers. The lightweight core mutually spaces apart the structural reinforcements, thereby increasing the flexural rigidity and reducing the overall weight of the structure. Cellular foams are easier to process than honeycomb panels and are the preferred core material when using a vacuum resin infusion process to impregnate structural fibre reinforcements with a resin matrix to form a lightweight sandwich panel.
There is currently a need for structural foams comprised of aromatic polyester, e.g. polyethylene terephthalate (PET), which exhibit a good balance of cost vs. mechanical properties such as compression strength, compression modulus, shear strength and shear modulus to enable the foams to be used as cores in sandwich panels comprising outer layers of a fibre reinforced matrix resin composite material. A drawback of current PET foams vs. other structural cores, such as PVC and SAN, used to form a fibre reinforced sandwich panels, is the increased resin absorption during vacuum infusion and prepreg processing vs. the more expensive structural cores. The increased resin absorption both increases the cost and weight of the final panel.
With an aim to reduce resin take-up by a polyethylene terephthalate (PET) core, the Applicants attempted to apply the sealing process of US-A-2005/0182239, which is limited to poly(meth)acrylamide foams, to polyethylene terephthalate (PET) foams. However, it was found that when the process of US-A-2005/0182239 was used on polyethylene terephthalate (PET) foams, although the resin take-up by the sealed surfaces was reduced, the peel strength between the surface of the core and the resin of the fibre reinforced matrix resin composite material was significantly reduced and fell below a minimum threshold required by a core in a structural sandwich panel. In addition the mechanical properties of the foam were reduced.
Consequently, despite the specific teaching of US-A-2005/0182239, which is limited to poly(meth)acrylamide foams, there is a need in the art for a method for treating polyethylene terephthalate (PET) foams in order to reduce the resin take-up by the foam surfaces when the foam is used as a core, while providing high mechanical properties of the foam and a high peel strength between the surface of the core and resin of a fibre reinforced matrix resin composite material bonded thereto by adhesion between the resin and the foam surface.